1. Field of the Invention
The present invention relates to an improved composite material reinforced by a para-oriented aramide fiber sheet, and to a process for preparing the same. More particularly, it relates to a composite material reinforced by a para-oriented aramide fiber sheet to have improved properties well suited for use as a composite material or reinforcing substrate for electrical applications, such as a printed circuit board or an insulating laminate.
2. Prior Art Statement
Para-oriented aramide fibers have improved physical properties, for example, they have excellent resistance to heat, resistance to fatigue and resistance to chemicals, and they have high strength and a high modulus of elasticity. However, since they have poor adhesiveness to thermosetting resins and cannot be intimately bonded to thermosetting resins using conventional technology, their excellent properties are not thoroughly utilized when they are used as substrates for composite materials. In order to overcome this problem, it has been proposed to expose the aramide fibers to plasma (as disclosed, for example, by Unexamined Japanese Patent Publication Nos. 125689/1984 and 171738/1986), to treat them with an aminopolyamide resin (as disclosed by Unexamined Japanese Patent Publication No. 204229/1986), to treat them with a polyurethane compound (as disclosed by Japanese Patent Publication No. 14166/1986) or to treat them with a resorcin/formalin oligomer (as disclosed by Unexamined Japanese Patent Publication No. 121591/1975). Although it is stated that the aramide fibers treated through these prior art techniques have improved adhesiveness, the results are not yet satisfactory.
In particular, a laminate for electrical uses should have high electric resistance even after it has absorbed water. However, a composite material reinforced by para-oriented aramide fibers and having satisfactory electric resistance in such a situation has not yet been known.
It has been known that a laminate can be prepared initially by impregnating a para-oriented aramide fiber sheet with a thermosetting resin, such as an epoxy resin, to form a prepreg which is then stacked and subjected to hot-press molding. The laminate prepared by such a process is low in thermal expansion coeficient and improved in dimensional stability due to the excellent properties of the para-oriented aramide fibers. (Unexamined Japanese Patent Publication No. 60500/1986)
However, the para-oriented aramide fibers in the resultant laminate do not adhere sufficiently to the thermosetting resin, which results in unsatisfactory electric resistance after it has absorbed water. Accordingly, the laminate prepared using conventional technology cannot be applied in practical uses.